Gallium Nitride based semiconductor laser diodes (LDs) are widely used in optical storage such as Blu-ray or high definition digital video discs (HD-DVD), as means of transferring, writing or reading data. As these devices are rapidly commercialised, there is a requirement for high power LDs, in particular for high-speed writing in these systems. However, at high power operation self-heating can affect the LD device characteristics, reducing the slope efficiency and maximum power output and increasing the threshold current. This effect can also reduce the operating lifetime. Therefore, a LD that can dissipate heat efficiently while maintaining low threshold current is required.
FIG. 1 is a cross sectional view of a conventional ridge waveguide structure of a semiconductor laser diode as described in U.S. Pat. No. 5,474,954 to Yang. The ridge structure formed on the upper cladding layer 6 defines the region whereby current is injected into the active layer 4, thus limiting the width of the resonance area. This ensures a stable transverse mode and low threshold current for lasing. From FIG. 1, the structure consists of an n-GaN substrate 1, an n-AlGaN cladding layer 2, an n-GaN lower waveguide layer 3, a InGaN active layer 4, a p-GaN upper waveguide layer 5, a p-AlGaN cladding layer 6 and a p-GaN contact layer 7. The refractive indices of the n-AlGaN cladding layer 2 and p-AlGaN cladding layer 6 are lower than that of the n-GaN waveguide layer 3 and p-GaN waveguide layer 5 respectively. In addition, the refractive indices of the n-GaN waveguide layer 3 and p-GaN waveguide layer 5 are lower than that of the InGaN active layer 4. An insulator layer, usually SiO2, is then formed on the sidewalls of the ridge (layer 8a) and on the etched surfaces at the bottom of the ridge (layer 8). A window is made on top of the ridge for the p-type upper electrode 9 to make electrical contact with the p-GaN contact layer 7. The lower n-type electrode is formed on the back of the n-GaN substrate 10. The poor thermal conductivity of SiO2 (k=1 W/mK) in this structure limits the heat dissipation properties of the laser diode under high power operation. This results in self-heating which limits the device lifetime.
In FIG. 2, a dual layer insulator structure is used to improve the laser diode heat dissipation properties as described in US2004/0218648A1 to Sung et al. For this structure, a protective layer 11 having a thermal conductivity higher than the buried layer is formed on top of the buried layer. High thermal conductivity insulators such as TiO2 (k˜25 W/mK) are typically used in this case. However, for p-side down mounted devices, the heat generated in this structure would initially still need to overcome the poor thermal conductivity SiO2 layer before reaching the high thermal conductivity protective layer. The protective layer also fully covers the buried.
FIG. 3 illustrates a high thermal conductivity and resistive MBE-grown AlxGa1-xN layer (k˜40-200 W/mK) 12 is formed as the insulator layer to improve the heat dissipation properties as discussed in GB0613890 to Hooper et al. However, the higher refractive index (RI) of the AlxGa1-xN layer (RI˜2.3) compared with the conventional SiO2 (RI˜1.5) results in reduced optical confinement, resulting in an increased threshold current compared to the current invention.
For high power laser diode applications, it is important that the excess heat generated by the device is efficiently dissipated. As discussed above, in general, good thermally conducting insulators have a high refractive index and using them as the sole insulator layer will degrade the optical confinement and increase the threshold current. This results in a higher dissipated power for the laser diode, which degrades the lifetime of the device. On the other hand, insulators with low refractive index, which are beneficial for good optical confinement, are generally poor thermal conductors, and are not suitable for high power devices.